"Sizing" refers to the treatment of either cellulosic pulp fibers or an already formed cellulosic material with chemicals which control the wettability and surface properties of the final cellulosic product.
Paper frequently is sized with resinates which are fixed to the cellulose fibers by means of aluminum sulphate (alum). Another well known sizing agent which is fixed by means of reacting with the hydroxyl groups of the cellulose is based on alkyl ketene dimers. This agent is usually added to the stock dispersed in a cationic emulsifier. Alkylketene dimers have certain advantages over resinates. One advantage is that the pH of the stock is kept within the range of 7 - 8 compared to a pH range of 4.5 - 6.5 which is used when the resinates are used as sizes -- which among other things imparts improved aging properties to the paper and makes it possible to use alkaline fillers, such as calcium carbonate. Furthermore, considerably lesser amounts of ketene dimers are required to obtain a certain degree of hydrophobicity and the sizing thus obtained is permanent. Moreover the paper will have improved resistance to lactic acid, ink, etc.
A disadvantage when employing alkylketene dimers is that no complete sizing effect is achieved until curing of the material for example by storing at room temperature for about 48 hours or by heating to a temperature of 100.degree.C for 20 minutes. Another disadvantage is that the sizing with alkylketene dimers is sensitive to alum present in the stock.
It also has been previously known to employ anhydrides of carboxylic acids as sizing agents, such as anhydrides of stearic acid or alkyl succinic acid. Said anhydrides do not have any of the above mentioned disadvantages that are encountered when employing resinates and ketene dimers, but greater amounts of the anhydride additives are required in order to achieve the same sizing effect as with the ketene dimers. In order to enhance the sizing effect it is common to add catalysts to the anhydrides so these catalysts are called sizing promoters and are used in relatively large amounts in combination with the anhydrides.